A multi-mode interference device (MMI) is generally in the form of a parallelepiped slab (body) of a material suitable for transmission of a selected optical wavelength. Such devices are used primarily in silicon on insulator (SOI) optical circuits. MMI devices and applications are described, for example, in “Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Applications”, Lucas B. Soldano and Erik C M Pennings, Journal Of Lightwave Technology, Vol. 13, No. 4, Apr. 1995.
FIG. 1 shows an optical power distribution obtained in an exemplary MMI device. The power levels are represented in a gray scale, from white for the lowest level, to black for the highest level. The center of the lower face of the device receives an optical power Pin supplied through a waveguide. The incident wave propagates omnidirectionally in the slab and is reflected on the faces of the slab, causing a stationary interference pattern. The pattern depends on the dimensions of the slab, on the wavelength, and on the refractive index. In various cross sections of the slab patterns called “modes” are formed, where power is evenly distributed among an integer number of equally spaced knots. FIG. 1 identifies several modes Mx, where x denotes the number of power nodes.
Thus, by suitably choosing the length of the slab, it is possible, in theory, to extract from the upper face a desired integer number of optical waves of the same power. In the example of FIG. 1, a 1-to-3 splitter (denoted 1×3) is achieved by choosing the length of the slab so that the interference pattern stops on a three-node mode M3.
The longitudinal position of the desired mode varies depending on the dimensions of the slab, the refractive index, and the wavelength. These parameters are dependent on temperature. When the MMI device is made of silicon and embedded with electronic control circuits, the dependence of the parameters on temperature is such that the device may become inoperative in temperature ranges usually encountered in electronic circuits.